1. Field of the Invention
The present invention relates to an ultrasonic motor which frictionally drives a moving member with vibrating member having a piezoelectric element, and to an electronic device using the ultrasonic motor. More particularly, the present invention relates to an ultrasonic motor used as a vibrating element to perform self-excited-oscillation driving.
2. Description of the Prior Art
Recently, ultrasonic motors as actuators based on a new principle and used in various electric devices have attracted attention and their applications in various fields, e.g., those to automatic focusing drives for cameras have been examined and developed. As drive systems for ultrasonic motors, separate excitation drive systems have generally been used in which a vibrating member having a piezoelectric element is excited in a predetermined vibration mode. Recently, since separate excitation drive systems have a complicated circuit configuration, self-excitation drive systems in which an ultrasonic motor is used as a vibrating element to cause oscillation at a resonance point of the vibrating element have been used generally on a trial basis instead of separate excitation drive systems, and practical self-excitation drive systems are now being developed. If a circuit according to such a system is used, the circuit can be formed so as to be simpler and smaller. This means that electronic devices incorporating drive circuits can be designed so as to be smaller in size and lower in price. Self-excited-oscillation drives for ultrasonic motors, e.g., one disclosed in Japanese Patent Examined Publication No. Hei 8-107686 have been proposed.
If a self-excited oscillator circuit such as described above is used to drive an ultrasonic motor, a function of changing the direction of movement of a moving member is achieved in such a manner that a drive signal is selectively applied to electrodes of a piezoelectric element to change the positional phase of a vibrational wave caused in the moving member. In such a case, however, there has been a problem in that the actually operating portion of the piezoelectric element used for driving is reduced, so that a sufficiently large output cannot be obtained.
It is, therefore, an object of the present invention to provide a self-excited oscillator circuit capable of changing the direction of movement of a moving member with respect to ultrasonic motors designed such that the entire of a piezoelectric element provided on a vibrating member is utilized to obtain a large output while the size of the motor is limited.
According to the present invention, self-excited-oscillation drive is realized with respect to a ultrasonic motor capable of changing the direction of movement of a moving member by using at least two groups of electrodes and by applying drive signals in phase with each other or in phase opposition to each other to the two electrode groups.
The present invention is characterized in that a common electrode is provided on one surface of a piezoelectric element; a plurality of drive electrodes are provided on the other surface of the piezoelectric element; the plurality of drive electrodes are connected to each other at one connection point by a connection means; and inverting means is provided between the connection point and at least one of the drive electrodes to invert the phase of a drive signal according to a command signal supplied from the outside, thereby enabling the direction of movement of a moving member to be changed. In this manner, self-excited-oscillation drive for the ultrasonic motor can be realized which enables the direction of movement of the moving member to be controlled by utilizing the entire of the piezoelectric element for drive.
The present invention is also characterized in that, in the above-described ultrasonic motor, an LC resonant circuit formed of the vibrating member and a capacitive element is utilized and the inverting means is provided between the vibrating member and the capacitive element. According to the present invention, a Colpitts oscillator circuit widely used in combination with quartz crystal is utilized to enable normal/reverse direction to be controlled in driving using the entire of a piezoelectric element.
The present invention is further characterized in that, in the above-described ultrasonic motor, a detection electrode is provided on another surface of the piezoelectric element separately from the plurality of drive electrodes, and a self-excited oscillator circuit is formed in such a manner that a detection signal obtained from the detection electrode is amplified by an amplifier circuit and output to the drive electrodes. According to the present invention, a vibration feedback type self-excited oscillator circuit is utilized to enable normal/reverse direction to be controlled in driving using the entire of a piezoelectric element.
The present invention is further characterized in that, in the above-described ultrasonic motor, the inverting means is formed by the amplifier circuit, and the level of the drive signal is constant irrespective of whether or not the phase of the drive signal is inverted. Therefore, it is possible to supply a large drive signal to the piezoelectric element while avoiding a loss in the drive signal in the inverting means, and to perform stable large-output drive. The moving member output is constant irrespective of the direction of movement of the moving member.
The present invention is further characterized in that a drive signal adjustment circuit is provided to adjust the drive signal so that the drive signal supplied to the drive electrode not connected to the inverting means, and the drive signal supplied to the drive electrode connected to the inverting means are substantially equal in level. The drive signals supplied to the two electrodes are thus equalized, so that the vibration caused in the vibrating member is balanced to achieve stable large-output drive.
Further, according to the present invention, an electronic device having the above-described ultrasonic motor can be obtained, and designs for smaller electronic devices become possible.